Even without the CA system, SSL (and TLS) is secure against _passive_ MITM attacks; That is, if the attacker cannot alter the data-stream. This is still reasona

Even without the CA system, SSL (and TLS) is secure against _passive_ MITM attacks; That is, if the attacker cannot alter the data-stream. This is still reasonable for many simple links, but for internet, it is not. If you've got no verification of the opposite party, literally any computer could be on the other end. The key exchange mechanism is secure, certainly, but any computer can execute it. The attacker can even show you the real website just proxied through, but that's plenty enough to get your password or any other access that the attacker wants.